1. Field of the Invention
The present invention relates to a controller for a vibration wave motor having an improved drive efficiency.
2. Description of the Prior Art
As shown in U.S. Pat. No. 4,019,073, a vibration wave motor transduces a vibration movement generated by applying a periodic voltage such as an AC voltage or a pulsating voltage to an electrostrictive element to a rotational movement or a one-dimensional movement. Since it does not need a winding unlike a conventional electromagnetic motor, it is simple in structure and compact, produces a high torque at a low rotating speed and has a low inertia in rotation.
However, the prior art vibration wave motor transduces a vibration movement of a standing wave generated in the vibration member to a unidirectional movement of the movable member by frictionally driving the movable member such as a rotor which contacts to the vibration member.
In order to reverse the direction of the movement, it is necessary to switch a mechanical construction such as switching of a contact position or a contact angle between the vibration member and the movable member. Accordingly, a large scale device is needed to reversibly drive the vibration wave motor, which sacrifices the advantage of the vibration wave motor, that is, the simple construction and the compactness.
As an approach to resolve the above problem, a vibration wave motor which is driven by a travelling vibration wave has recently been proposed.
FIG. 1 illustrates components of such a vibration wave motor.
Fitted in a central cylinder 5a of a stator 5 which serves as a base are vibration absorber 4, a metal ring vibration member 2 having an electrostrictive device 3 bonded on a surface facing the absorber 4, and a movable member 1, in this order. The stator 5, the absorber 4 and the vibration member 2 are mounted to prevent relative rotation. The movable member 1 is press-contacted to the vibration member 2 by its weight or biasing means, not shown, to maintain an integrity of the motor. The electrostrictive device 3 includes a group of electrostrictive elements 3A.sub.1 -3A.sub.7 arranged at a pitch equal to one half of a wavelength .lambda. of a vibration wave. The electrostrictive elements 3A.sub.1, 3A.sub.3, 3A.sub.5 and 3A.sub.7 are polarized in one direction and the interposing electrostrictive elements 3A.sub.2, 3A.sub.4 and 3A.sub.6 are polarized in the opposite direction. Thus, the electrostrictive elements 3A.sub.1 -3A.sub.7 are polarized oppositely between the adjacent ones. The electrostrictive device 3 includes another group of electrostrictive elements 3B.sub.1 -3B.sub.7 which are also arranged at the pitch of .lambda./2 and polarized oppositely between adjacent ones.
The group of electrostrictive elements 3A.sub.1 -3A.sub.7 and the group of electrostrictive elements 3B.sub.1 -3B.sub.7 are phase-differentially arranged at a mutual pitch of (.eta.o+1/4).lambda., where .eta.o=0, 1, 2, 3, . . . .
The electrostrictive device 3 need not be the plurality of electrostrictive elements but it may be a single ring element 3 which is polarized at the pitch of .lambda./2 to form polarized areas 3a.sub.1 -3a.sub.5 and 3b.sub.1 -3b.sub.5, as shown in FIG. 2.
A lead wire 11a is connected to the electrostrictive elements 3A.sub.1 -3A.sub.7. On the sides facing the absorber 4, and a lead wire 11b is connected to the electrostrictive elements 3B.sub.1 -3B.sub.7, and those wires are connected to a power supply 6a and a 90.degree. phase shifter 6b (see FIG. 3). The lead wire 11c is connected to the metal vibration member 2 and to the AC power supply 6a.
The vibration wave motor thus constructed operates in the following manner.
FIG. 3 illustrates the generation of the vibration wave in the motor. While the electrostrictive elements 3A.sub.1 -3A.sub.4 and 3B.sub.1 -3B.sub.4 are shown in adjacent to each other for the purpose of explanation, they meet the requirement of .lambda./4 phase shift and they are essentially equivalent to the arrangement of the electrostrictive elements 3A.sub.1 -3A.sub.4 and 3B.sub.1 -3B.sub.4 of the motor shown in FIG. 1. Symbols .sym. in the electrostrictive elements 3A.sub.1 -3A.sub.4 and 3B.sub.1 -3B.sub.4 indicate that the electrostrictive elements expand in a positive cycle of the AC voltage and symbols .crclbar. indicate that they shrink in the positive cycle.
The metal vibration member 2 is used as one electrode for the electrostrictive elements 3A.sub.1 -3A.sub.4 and 3B.sub.1 -3B.sub.4, the AC voltage of V=Vo sin .omega.t is applied to the electrostrictive elements 3A.sub.1 -3A.sub.4 from the AC power supply 6a and the .lambda./4-phase shifted AC voltage of V=Vo sin (.omega.t.+-..pi./2) is applied to the electrostrictive elements 3B.sub.1 -3B.sub.4 from the AC power supply 6a through the 90.degree. phase shifter 6b, where signs + and - in the equation are selected by the phase shifter 1 depending on the direction of movement of the movable member 1 (not shown in FIG. 3). When the sign + is selected, the phase is shifted by +90.degree. and the movable member 1 is moved forward, and when the symbol - is selected, the phase is shifted by -90.degree. and the movable member 1 is moved reversely. It is now assumed that the sign - is selected and the voltage of V=Vo sin (.omega.t-.pi./2) is applied to the electrostrictive elements 3B.sub.1 -3B.sub.4. When only the electrostrictive elements 3A.sub.1 -3A.sub.4 are vibrated by the application of the voltage of V=Vo sin .omega.t, a vibration by a standing wave as shown in FIG. 3(a) is generated. When only the electrostrictive elements 3B.sub.1 -3B.sub.4 are vibrated by the application of the voltage of V=Vo sin (.omega.t-.pi./2), a vibration by a standing wave as shown in FIG. 3(b) is generated. When the two AC voltages having a phase difference therebetween are simultaneously applied to the electrostrictive elements 3A.sub.1 -3A.sub.4 and 3B.sub.1 -3B.sub.4, a vibration wave is a travelling wave. FIG. 3(c) shows a waveform at a time t=2n.pi./.omega., FIG. 3(d) shows a waveform at a time t=.pi./2.omega.+2n.pi./.omega., FIG. 3(e) shows a waveform at t=.pi./.omega.+2n.pi./.omega. and FIG. 3(f) shows a waveform at t=3.pi./2.omega.+2n.pi./.omega.. A wavefront of the vibration wave travels in the direction of an x-axis.
The travelling wave includes a longitudinal wave and a lateral wave. Looking at a mass point of the vibrating member 2 as shown in FIG. 4, a longitudinal amplitude u and a lateral amplitude w cause a clockwise rotating elliptic movement. The movable member 1 is press-contacted to the surface of the vibrating member 2 and it contacts to only an apex of the vibration plane. Thus, the vibration member 1 is driven by the component of the longitudinal amplitude u of the elliptic movement of the mass points A, A', . . . at the apex so that it is moved in a direction of an arrow N.
The velocity of the mass point A at the apex is V=2.pi.fu (when f is the vibration frequency) and the velocity of movement of the movable member 1 depends on the velocity of the mass point and also depends on the lateral amplitude w because of the frictional drive by the press-contact. Thus, the velocity of the movable member 1 is proportional to the magnitude of the elliptic movement of the mass point A.
On the other hand, since the movable member 1 is friction-driven at the apex of the wavefront of the travelling vibration wave of the vibration member 2, it is necessary that the wavefront in the direction of apex (z-axis direction in FIG. 4) resonates in order to improve a drive efficiency. There exists a relationship of f=.sqroot.E/3.rho..multidot..pi.h/.lambda..sup.2, where f (=2.pi..omega.) is the frequency of the input voltage, E is a Young's modulus of the vibration member 2, .rho. is a density, h is a thickness and .lambda. is a wavelength of the wave generated. The resonance occurs at the thickness which meets the above relationship.
Since the vibration member 2 is of ring shape, the travelling vibration wave travels along the ring and the resonance occurs when the newly generated wave and the circumferential length .pi.D is n times (where n is a natural number) as large as the wavelength .lambda., that is, n.lambda.=.pi.D.
The amplitude of the travelling vibration wave grows by the resonance so that the drive efficiency of the vibration wave motor is improved.
In order to improve the drive efficiency of such vibration wave motor, it is necessary to control the frequency of the applied periodic voltage while taking various conditions such as the thickness and the radius of the vibration member into consideration.
However, after the adjustment, the resonance frequency may shift or vary by a change of a temperature of the motor or an oscillation circuit, or a fatigue of the motor such as abrasion of the vibration member.
In addition, a manufacturing cost increases because of adjustment step of the frequency.
On the other hand, it is desirable that the frequency is controlled such that an optimum resonance frequency is attained at a highest rotating speed of the vibration wave motor.
It is an object of the present invention to provide a controller for a vibration wave motor which stores a frequency which results in a highest measured drive speed of the vibration wave motor and drives the motor by a periodic voltage of the stored frequency in order to improve a drive efficiency.
It is another object of the preset invention to provide a controller for a vibration wave motor which intermittently drives the motor by a periodic voltage of a stored frequency.
It is other object of the present invention to provide a controller for a vibration wave motor which stores a frequency which results in a maximum measured amplitude of a vibration member of the vibration wave motor and drives the motor by a periodic voltage of the stored frequency.